1. Field of the Invention
The present invention relates generally to a compressor which compresses fluid, introduced into cylinder bores, by reciprocating pistons. More particularly, it pertains to a wave cam plate type compressor which reciprocates pistons by rotating a wave cam integrally attached to a drive shaft.
2. Description of the Related Art
In the prior art, swash plate type compressors are provided with a drive shaft, a swash plate, and pistons accommodated in associated cylinder bores. The swash plate is integrally fixed to the drive shaft and connected to each piston. In this type of compressor, fluid introduced into the cylinder bores is compressed by reciprocating movement of the pistons within the bores. The reciprocation is caused by integral rotation of the drive shaft and the wave cam. In this compressor, a movement diagram indicating axial displacement of a point following the swash plate surface during one rotation of the swash plate shows a single cycle sine wave curve. Hence, one compression stroke is performed per rotation of the drive shaft in the swash plate type compressor.
Wave cam type compressors have been developed to provide a compressor with a smaller size and an increased discharge volume as compared to swash plate type compressors. The wave cam type compressors are provided with a drive shaft, a wave cam, and pistons accommodated in associated cylinder bores. The wave cam is integrally fixed to the drive shaft and connected to each piston. In this type of compressor, fluid introduced into the cylinder bores is compressed by reciprocating movement of the pistons within the bores. The reciprocation is caused by integral rotation of the drive shaft and the wave cam. In the wave cam type compressor, a movement diagram indicating axial displacement of a point following the wave cam surface during one rotation of the wave cam shows a double cycle sine wave curve. Hence, two compression strokes are performed per rotation of the drive shaft in the wave cam type compressor. Thus, a wave type cam compressor has a larger discharge volume and a smaller size than a swash plate type compressor.
An example of such a wave cam type compressor is disclosed in Japanese Unexamined Patent Publication No. 57-110783. This compressor employs a wave cam having a front and a rear surface, and double-headed pistons having heads on its two ends. A roller, interposed between each cam surface and each piston, is rotatably and permanently fitted within the piston. Rotation of the wave cam moves the roller relatively with respect to the wave cam surfaces, axially displacing the contact point between the roller and the piston to reciprocate the pistons. The reciprocation of the pistons is based on a curve of the cam surface.
As shown in FIG. 9, a prior art wave cam 80 has a cam surface 81 including concave surfaces 81a and convex surfaces 81b. The surfaces 81a, 81b are formed continuously. When the center points of the concave surfaces 81a are aligned with a piston (not shown), the piston is located at a bottom dead center position. When the center points of the convex surfaces 81b are aligned with the piston, the piston is located at a top dead center position.
The cam surface 81 of the wave cam 80 shown in FIG. 9 reciprocates pistons via rollers (not shown). Therefore, the cam surface 81 of the wave cam 80 requires high precision grinding. To grind the cam surface 81, the wave cam 80 is rotated in one direction while a grinding stone 84, disposed parallel to the cam surface 81, is rotated by a shaft 83.
However, the shape of the cam surface 81 having continuous concave and convex surfaces 81a, 81b causes problems described below during its grinding.
FIGS. 10 and 11 show the cam surface 81 which is to be ground by a grinding stone 84. FIG. 10 shows a contact area .alpha. between the cam surface 81 and the grinding stone 84 during grinding of the convex surface 81b. FIG. 11 shows a contact area .beta. between the cam surface 81 and the grinding stone 84 during grinding of the concave surface 81a. As apparent from these drawings the contact area .alpha. is different from the contact area .beta.. Therefore, grinding conditions differ between the concave and convex surfaces 81a, 81b. This lowers grinding accuracy, especially at the boundary portions between the concave and convex surfaces 81a, 81b, and may result in the cam surface 81 having inconsistent surface roughness and dimensions. As a result, rolling of the rollers between the wave cam 80 and the pistons may be rough and may cause a decrease in compressing efficiency of the compressor.
To cope with these problems, a wave cam 91 having a cam surface 92 that is entirely a convex surface 92a, as shown in FIG. 12, may be used. The contact area with a grinding stone is substantially equal at all points along the entire circumference of the cam surface 92.
However, the wave cam 91 may decrease grinding ability of the grinding stone due to ground dust clogging a grinding surface of the grinding stone. When the grinding surface becomes clogged, it is necessary to increase the pressing force of the grinding stone on the cam surface 92 to ensure the same predetermined grinding ability while continuously using the same grinding stone. The reaction force acting on the grinding stone becomes large when the pressing force is increased. Accordingly, when the grinding stone 84 shown in FIG. 9 is used to grind the cam surface 92, the pressing force deflects its shank 83. This leads to unsatisfactory contact between the grinding stone 84 and the cam surface 92 thus decreasing the grinding accuracy of the grinding stone 84 on the cam surface 92.
Furthermore, a plurality of wave cams 91 are successively ground by a single grinding stone in the manufacturing process. Hence, a grinding stone which may be used for a long period of time is desirable in the view point of manufacturing efficiency of the wave cam 91. Accordingly, a wave cam 91 having a cam surface 92 capable of prolonging the tool life of the grinding stone is desired.